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[dm-devel] Re: [PATCH 3/3] Add timeout feature


>> - struct super_block *sb = freeze_bdev(mp->m_super->s_bdev);
>> + struct super_block *sb = freeze_bdev(mp->m_super->s_bdev, 0);
> Using NULL here is clearer and will, I expect, avoid a sparse warning.

I checked it but I couldn't find a sparse warning in xfs_fsops.c.
Can you tell me how to use NULL?

struct super_block *sb = freeze_bdev(mp->m_super->s_bdev, NULL);


It's much better to use NULL here rather than literal zero because the
reader of this code can then say "ah-hah, we're passing in a pointer". Whereas plain old "0" could be a pointer or a scalar.

The second argument's type of freeze_bdev() is "long", not pointer as below.
struct super_block *freeze_bdev(struct block_device *, long timeout_msec);

So "0" is reasonable, isn't it?

We should always use NULL to represent a null pointer in the kernel. The one acceptable exception is when testing for nullness:

if (ptr1)
if (!ptr2)

Often people will use

if (ptr1 != NULL)
if (ptr2 == NULL)

in this case as well.  (I prefer the shorter version personally, but
either is OK).

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